Glatiramer Acetate
Copolymer-1, also known as glatiramer acetate and marketed under the tradename Copaxone®, comprises the acetate salts of polypeptides containing L-glutamic acid, L-alanine, L-tyrosine and L-lysine. The average molar fractions of the amino acids are 0.141, 0.427, 0.095 and 0.338, respectively, and the average molecular weight of copolymer-1 is between 4,700 and 11,000 daltons. Chemically, glatiramer acetate is designated L-glutamic acid polymer with L-alanine, L-lysine and L-tyrosine, acetate (salt). Its structural formula is:(Glu, Ala, Lys, Tyr)xCH3COOH (C5H9NO4—C3H7NO2—C6H14N2O2—C9H11NO3)xC2H4O2 [CAS—147245-92-9],approx. ratio Glu14Ala43Tyr10Lyz34x(CH3COOH)20. Copaxone® is a clear, colorless to slightly yellow, sterile, nonpyrogenic solution for subcutaneous injection. Each milliliter contains 20 mg of glatiramer acetate and 40 mg of mannitol. The pH range of the solution is approximately 5.5 to 7.0.Mechanism of Action
Glatiramer acetate is a random polymer (average molecular mass 6.4 kD) composed of four amino acids that are found in myelin basic protein. The mechanism of action for glatiramer acetate is unknown, although some important immunological properties of this copolymer have emerged. Administration of copolymer-1 shifts the population of T cells from pro-inflammatory Th1 cells to regulatory Th2 cells that suppress the inflammatory response (FDA Copaxone® label). Given its resemblance to myelin basic protein, copolymer-1 may also act as a decoy, diverting an autoimmune response against myelin. The integrity of the blood-brain barrier, however, is not appreciably affected by copolymer-1, at least not in the early stages of treatment.
Copolymer-1 is a non-autoantigen which has been demonstrated to suppress experimental allergic encephalomyelitis (EAE) induced by various encephalitogens including mouse spinal cord homogenate (MSCH) which includes all myelin antigens, such as myelin basic protein (MBP) (Sela M et al., Bull Inst Pasteur (1990) 88 303-314), proteolipid protein (PLP) (Teitelbaum D et al., J Neuroimmunol (1996) 64 209-217) and myelin oligodendrocyte glycoprotein (MOG) (Ben-Nun A et al., J Neurol (1996) 243 (Suppl 1) S14-S22) in a variety of species. EAE is an accepted model for multiple sclerosis.
Copolymer-1 has been demonstrated to be active when injected subcutaneously, intraperitoneally, intravenously or intramuscularly (Teitelbaum D et al., Eur J Immunol (1971) 1 242-248; Teitelbaum D et al., Eur J Immunol (1973) 3 273-279). In phase III clinical trials, daily subcutaneous injections of copolymer-1 were found to slow the progression of disability and reduce the relapse rate in exacerbating-remitting multiple sclerosis (Johnson K P, Neurology (1995) 1 65-70; www.copaxone.com). Copolymer-1 therapy is presently limited to daily subcutaneous administration. Treatment with copolymer-1 by ingestion or inhalation is disclosed in U.S. Pat. No. 6,214,791, but these routes of administration have not been shown to attain clinical efficacy in human patients.
Efficacy
Evidence supporting the effectiveness of glatiramer acetate in decreasing the frequency of relapses in patients with Relapsing-Remitting Multiple Sclerosis (RR MS) derives from two placebo-controlled trials, both of which used a glatiramer acetate dose of 20 mg/day. No other dose or dosing regimen has been studied in placebo-controlled trials of RR MS (www.copaxone.com). A comparative trial of the approved 20 mg dose and the 40 mg dose showed no significant difference in efficacy between these doses (The 9006 trial; Cohen J A et al., Neurology (2007) 68 939-944). Various clinical trials in glatiramer acetate are on-going. These include studies with a higher dose of glatiramer acetate (40 mg—the FORTE study); studies in Clinically Isolated Syndrome patients (the PreCISe study) as well as numerous combination and induction protocols, in which glatiramer acetate is given together with or following another active product.
Side Effects
Currently, all specifically approved treatments of multiple sclerosis involve self injection of the active substance. Frequently observed injection-site problems include irritation, hypersensitivity, inflammation, pain and even necrosis (in the case of interferon 1β treatment) and a low level of patient compliance.
Side effects generally include a lump at the injection site (injection site reaction), aches, fever, and chills. These side effects are generally mild in nature. Occasionally a reaction occurs minutes after injection in which there is flushing, shortness in breath, anxiety and rapid heartbeat. These side effects subside within thirty minutes. Over time, a visible dent at the injection site due to the local destruction of fat tissue, known as lipoatrophy, may develop. Therefore, an alternative method of administration is desirable.
More serious side effects have been reported for glatiramer acetate, according to the FDA's prescribing label, these include serious side effects to the body's cardiovascular system, digestive system (including liver), hemic and lymphatic system, musculoskeletal system, nervous system, respiratory system, special senses (in particular the eyes), urogenital system; also reported have been metabolic and nutritional disorders; however a link between glatiramer acetate and these adverse effects has not been definitively established (FDA Copaxone® label).
Depot Systems
The parenteral route by intravenous (IV), intramuscular (IM), or subcutaneous (SC) injection is the most common and effective form of delivery for small as well as large molecular weight drugs. However, pain, discomfort and inconvenience due to needle sticks makes this mode of drug delivery the least preferred by patients. Therefore, any drug delivery technology that can at a minimum reduce the total number of injections is preferred. Such reductions in frequency of drug dosing in practice may be achieved through the use of injectable depot formulations that are capable of releasing drugs in a slow but predictable manner and consequently improve compliance. For most drugs, depending on the dose, it may be possible to reduce the injection frequency from daily to once or twice monthly or even longer (6 months). In addition to improving patient comfort, less frequent injections of drugs in the form of depot formulations smoothes out the plasma concentration-time profile by eliminating the hills and valleys. Such smoothing out of plasma profiles has the potential to not only boost the therapeutic benefit in most cases, but also to reduce any unwanted events, such as immunogenicity etc. often associated with large molecular weight drugs.
Microparticles, implants and gels are the most common forms of biodegradable polymeric devices used in practice for prolonging the release of drugs in the body. Microparticles are suspended in an aqueous media right before injection and one can load as much as 40% solids in suspensions. Implant/rod formulations are delivered to SC/IM tissue with the aid of special needles in the dry state without the need for an aqueous media. This feature of rods/implants allows for higher masses of formulation, as well as drug content to be delivered. Further, in the rods/implants, the initial burst problems are minimized due to much smaller area in implants compared to the microparticles. Besides biodegradable systems, there are non-biodegradable implants and infusion pumps that can be worn outside the body. Non-biodegradable implants require a doctor's visit not only for implanting the device into the SC/IM tissue but also to remove them after the drug release period.
Injectable compositions containing microparticle preparations are particularly susceptible to problems. Microparticle suspensions may contain as much as 40% solids as compared with 0.5-5% solids in other types of injectable suspensions. Further, microparticles used in injectable depot products, range in size up to about 250 μm (average, 60-100 μm), as compared with a particle size of less than 5 μm recommended for IM or SC administration. The higher concentrations of solids, as well as the larger solid particle size require larger size of needle (around 18-21 gauge) for injection. Overall, despite the infrequent uses of larger and uncomfortable needles, patients still prefer less frequently administered dosage forms over everyday drug injections with a smaller needle.
Biodegradable polyesters of poly(lactic acid) (PLA) and copolymers of lactide and glycolide referred to as poly(lactide-co-glycolide) (PLGA) are the most common polymers used in biodegradable dosage forms. PLA is hydrophobic molecule and PLGA degrades faster than PLA because of the presence of more hydrophilic glycolide groups. These biocompatible polymers undergo random, non-enzymatic, hydrolytic cleavage of the ester linkages to form lactic acid and glycolic acid, which are normal metabolic compounds in the body. Resorbable sutures, clips and implants are the earliest applications of these polymers. Southern Research Institute developed the first synthetic, resorbable suture (Dexon®) in 1970. The first patent describing the use of PLGA polymers in a sustained release dosage form appeared in 1973 (U.S. Pat. No. 3,773,919).
Today, PLGA polymers are commercially available from multiple suppliers; Alkermes (Medisorb polymers), Absorbable Polymers International [formerly Birmingham Polymers (a Division of Durect)], Purac and Boehringer Ingelheim. Besides PLGA and PLA, natural cellulosic polymers such as starch, starch derivatives, dextran and non-PLGA synthetic polymers are also being explored as biodegradable polymers in such systems.
At present no long acting dosage forms of glatiramer acetate are available. This is a huge unmet medical need, as these formulations would be extremely beneficial to many patients, particularly to those with neurological symptoms or physical disabilities.